Table 4: Optical depthsa.
Line Observed Model  Model 
    I II
110-111 (O) $0.025\pm0.007$ 0.028 0.036
211-212 (O) $0.006\pm0.002$ 0.010 0.007
212-111 (O) $0.493\pm0.090$ 0.436 0.183
211-110 (O) $0.890\pm0.151$ 0.426 0.160
101-000 (P) $0.331\pm0.068$ 0.257 0.250
202-101 (P) $0.311\pm0.057$ 0.352 0.346

$\chi^2_{\rm reduced, ortho}$		 - 7.1 18.9
$\chi^2_{\rm reduced, para}$ - 1.7 1.8
N(H2CO) - 3.9 2.6
Ortho/Para - 2.2 0.9
a Column 1: H2CO transition with O and P denoting ortho- and para-H2CO, respectively. The upper two lines refer to K-doublet transitions. In the lower part of the Table, reduced $\chi^2$ values, total H2CO column densities from the +7 km s-1 component in units of 1013 cm-2, and H2CO ortho-to-para abundance ratios are given. Column 2: observed apparent optical depths taken from the last column of Table 3. Given errors are larger than those in Table 3, also accounting for a calibration uncertainty of $\pm$15%. Column 3: LVG results exclusively accounting for microwave background excitation ( $T_{\rm CMB}$ = 4.6 K =  $T_{\rm ex}$; Model I). Column 4: ame as Col. 3, but including collisional excitation (n(H2) = 500 cm-3; Model II). For details, see Sect. 4.2.3.

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